An oscilloscope is used to provide a visual representation of the waveform of a signal. In the case of an analog oscilloscope, the horizontal sweep is initiated in response to a trigger pulse, which may be generated when the voltage of the input signal passes through a predetermined trigger level in a selected direction, i.e. positive going or negative going. Therefore, the time of occurrence of the trigger pulse is implicit in the information provided on the oscilloscope screen.
A digital storage oscilloscope (DSO) has two distinct modes of operation. In the acquisition mode, the DSO acquires an analog input signal by sampling the input signal at predetermined times, quantizing the successive samples so as to generate a succession of digital words representative of the magnitudes of the respective samples, and writing the digital words into memory locations that are related to the respective sample times. The DSO responds to a trigger pulse by initiating a sequence of operations associated with terminating the signal acquisition. In the display mode, the contents of the waveform memory are used to generate a display, with greater or lesser resolution in the time dimension, of the waveform of the signal during the interval over which the acquisition took place. The display interval (that portion of the acquisition interval which is displayed) may include the trigger point, but even if it does, the position of the trigger point has no necessary relationship to the position of the waveform displayed on the oscilloscope screen. It is conventional for a DSO to include a text generator that causes an alphanumeric representation of the operating mode of the oscilloscope to be displayed on the oscilloscope screen. In particular, this alphanumeric information may include the trigger voltage level. Although the numerical value of the trigger voltage is displayed, this does not provide immediate visual identification of the trigger point with reference to features of the displayed waveform.